The invention relates to a picture display device comprising a cathode ray tube having in an evacuated envelope centred along an electron-optical axis a cathode and a number of lens electrodes which together constitute the electron gun for producing an electron beam. The gun has an accelerating electrostatic electron lens formed by the last two lens electrodes on the display screen side of the electron gun for focusing the electron beam onto a display screen. The cathode ray tube is surrounded by a system of deflection coils for deflecting the electron beam from the axis and across the display screen. The system of deflection coils surrounds the electron lens.
Such a picture display device is known from U.S. Pat. No. 2,151,777 in which it is suggested to place the electrostatic electron lens for focusing the electron beam onto the display screen at least partly inside the system of deflection coils to obtain a shorter device. In this case the deflection point of the system of deflection coils lies between the two major faces of the electrostatic electron lens. The location of the two major faces of the electrostatic electron lens is easy to determine from the tables in "Electrostatic Lenses", E. Harting and F. H. Read, Elsevier Scientific Publishing Company, Amsterdam-Oxford-New York 1976. This location will be further described hereinafter.
The system of deflection coils, however, has a number of electron-optical aberrations of which the most obvious are the astigmatism and the curvature of field. Curvature of field is the non-coincidence of the main surface of the image (or sometimes "surface of best focus") with the display screen. Whereas the astigmatism can be corrected substantially entirely by a correct choice of the design of the deflection coils, the radius of curvature of the main surface of the image is approximately equal to ##EQU1## wherein k is the effective length of the deflection field of the deflection coils and L is the distance from the deflection point to the display screen. This deflection point is located on the electron-optical axis of the electron gun and is the point of intersection with said axis of a plane perpendicular to said axis from which, in the case of maximum deflection of the electron beam, the electrons appear to originate when viewed from the display screen. The place of said deflection point on the axis will be described in greater detail hereinafter.
It is possible to correct the curvature of field by means of dynamic focusing. The strength of the electron lens for focusing the electron beam, which lens is sometimes termed focusing lens, is adjusted as a function of the deflection to which the electron beam is subjected at that instant. As a result of this it is possible to continuously cause the main surface of the image to intersect the display screen where the electron beam impinges on the display screen. However, this manner of correcting necessitates the incorporation of an extra circuit for generating the correct dynamic focusing voltages at the electrodes of the focusing lens.